1. Field of the Invention
The present invention relates to spectral purity filters, lithographic apparatus including such spectral purity filters, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In addition to Extreme Ultra-Violet (EUV) radiation, an EUV source emits many different wavelengths of light and debris. This non-EUV radiation may be harmful for the EUV lithography system, so it has to be removed by a spectral purity filter. Present spectral purity filters are based on blazed gratings. These gratings may be difficult to produce, since the surface quality of the triangular shaped pattern has to be very high. The roughness of the surface should be lower than 1 nm RMS. Debris mitigation schemes are also applied for suppressing the debris. However, debris mitigation may be problematic as debris mitigation methods which include foil traps and gas buffers may not guarantee effective debris protection. Moreover, use of usual (e.g. Zr) thin filters transmissive for EUV is difficult due to the fragility of the filters and low heat-load threshold. In addition the glue used for filters on mesh is not desirable for H-vacuum systems.
A further problem with existing spectral purity filters is that they change the direction of the light from the EUV source. Therefore, if a spectral purity filter is removed from an EUV lithography apparatus, a replacement spectral purity filter has to be added or a mirror at a proper angle has to introduced. The added mirror introduces unwanted losses into the system.
U.S. Pat. No. 6,456,362, incorporated herein by reference, discloses waveguides for use in EUV lithographic projection apparatus which describes an integrator based on waveguides.
U.S. Pat. No. 6,809,327, incorporated herein by reference, discloses an apparatus including a plasma source to generate a spectrum of radiation that includes EUV radiation, a reflector to generate a beam of EUV radiation from the spectrum of radiation, and a thin film to pass at least a portion of the EUV radiation.